The present invention relates to a vulcanizable rubber composition which can be used for the manufacture of tires and a process for the preparation of this composition. The invention also provides a tire, the tread of which comprises such a composition. The rubber composition of the invention possesses improved hysteresis and physical properties in the vulcanized state, while retaining satisfactory processing properties in the non-vulcanized state.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce rubber mixes having good mechanical properties and as low a hysteresis as possible so that they can be used in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, sidewalls or treads, and in order to obtain tires having reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, in particular those that modify the structure of diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or starring agents. The very great majority of these solutions have concentrated on the use of functionalized polymers which are active with respect to carbon black, with the aim of obtaining a good interaction between the polymer, thus modified, and the carbon black.
By way of illustration, the prior art relating to reinforcing fillers formed of carbon black, includes, for example, U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the end of a chain with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as coupling or starring agent for diene polymers.
Silica has long been used as a reinforcing filler in vulcanizable rubber compositions, in particular those intended for use in tire treads. However, this use has remained very limited, because of an unsatisfactory level of certain physical properties of such compositions, in particular abrasion resistance.
In order to overcome these drawbacks, the use functionalized diene polymers instead of the non-functionalized polymers which were used before has been proposed. In particular, polymers functionalized by alkoxysilane derivatives, such as tetra-ethoxysilanes, has been proposed. For example, U.S. Pat. No. 5,066,721 describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolyzable alkoxy radical, which makes it possible to eliminate the polymerization solvent by steam stripping.
One disadvantage of these functionalization reactions relates to the coupling reactions which accompany them, which generally makes it necessary to use an excess of alkoxysilane and/or intensive mixing, in order to minimize these coupling reactions.
Another drawback of these reactions involves the later implementation of the steam stripping operation, which is necessary to eliminate the polymerization solvent.
Generally, experience has shown that the functionalized polymers obtained in this fashion undergo changes in macrostructure during the stripping operation, which results in serious degradation of their properties, unless the functionalizing agent is an alkoxysilane belonging to a restricted family, such as that described in U.S. Pat. No. 5,066,721.
Consequently, the use of diene polymers comprising an alkoxysilane function to produce rubber compositions comprising silica as reinforcing filler has not proven satisfactory, despite the improved physical properties of these compositions.
This is why further research has been carried out on other functionalization reactions with the view to obtaining such rubber compositions.
For example, French patent specification FR-A-2 740 778 of the assignee of the present inventors discloses the use of diene polymers bearing a silanol function or a polysiloxane block having a silanol end at the chain end. For example, a cyclic polysiloxane functionalizing agent is used, such as hexamethylcyclotrisiloxane. The functionalized polymers obtained can be separated from the reaction medium resulting in their formation by steam extraction of the solvent, without their macrostructure and, consequently, their physical properties changing.
These functionalized polymers are intended to be incorporated in vulcanized compositions comprising silica as the majority reinforcing filler, including fillers comprising a blend of silica and carbon black. It has been possible to establish that these polymers impart rubber properties, in particular hysteresis and reinforcement properties in the vulcanized state, which are improved compared with those of control compositions based on non-functionalized diene polymers. Such compositions are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane function.
However, this improvement of the hysteresis and reinforcement properties in the vulcanized state is accompanied by an increase in the Mooney viscosity of the compositions tested relative to control compositions, i.e., by a reduced ability: to be worked in the non-vulcanized state.